Aquatic chemistry is becoming both a rewarding and substantial area of inquiry and is drawing many prominent scientists to its fold. Its literature has changed from a compilation of compositional tables to studies of the chemical reactions occurring within the aquatic environments. But more than this is the recognition that human society in part is determining the nature of aquatic systems. Since rivers deliver to the world ocean most of its dissolved and particulate components, the interactions of these two sets of waters determine the vitality of our coastal waters. This significant vol ume provides not only an introduction to the dynamics of aquatic chem istries but also identifies those materials that jeopardize the resources of both the marine and fluvial domains. Its very title provides its emphasis but clearly not its breadth in considering natural processes. The book will be of great value to those environmental scientists who are dedicated to keeping the resources of the hydrosphere renewable. As the size of the world population becomes larger in the near future and as the uses of materials and energy show parallel increases, the rivers and oceans must be considered as a resource to accept some of the wastes of society. The ability of these waters and the sediments below them to accommodate wastes must be assessed continually. The key questions relate to the capacities of aqueous systems to carry one or more pollutants."

Metal pollution in the aquatic environment

More insight on rare earth element pathways from land to ocean comes from analyses of suspended matter in four major world rivers (Amazon, Congo, Ganges, Mekong), while their geochemical behavior at the river-ocean boundary has been studied in the Gironde estuary (France). The Gironde data indicate that soluble rare earth elements are removed from solution in the estuarine zone; this observation invalidates any rare earth element balance to the ocean which would not take into account the river-ocean boundary processes. Data on rare earth element concentrations in soluble and detrital river loads show that solid discharge represents the major part of rare earth elements supplied by rivers to the ocean. A rare earth material balance to the ocean is discussed on the basis of the results presented in this paper.

Rare earth element supply to the Ocean

The distribution of suspended particle size and concentration were measured along the Gironde Estuary, France, from the river seaward to the ocean. The suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage.



Sediments discharged by the rivers coagulate upon encountering the very low salinities (0.2‰ of the upper estuary (confirmed with laboratory experiments), and are then transported and deposited by currents in the remainder of the estuary. This coagulation, coupled with estuarine circulation, produces a turbidity maximum which is offset between the surface and bottom waters. The floc size maximum is oceanward of the turbidity maximum and is, likewise, offset along the estuary by about 30 km. The estuary can be subdivided into the following zones: (1) coagulation; (2) hydrodynamic, landward of the null point; and (3) hydrodynamic, seaward of the null point. Initial coagulation appears to be completed in coagulation zone (1), and particles are transported and settled (with very little floc breakage and recoagulation) in zones (2) and (3) only. The floc settling velocities, coupled with estuarine circulation, control the concentration and size distributions of flocs in the water column, and eventually control the deposition of sediments.

Coagulation and transport of sediments in the Gironde Estuary

Suspended matter and bottom deposits of the Orinoco delta: Turbidity, mineralogy and elementary composition

Investigations on dissolved uranium, silicon and on particulate trace elements in estuaries

SUMMARY

The determination of the natural radioelements in suspended matter in rivers and estuaries, lead to a simple relation between gamma activity and turbidity T, such as Nt=aTb (Nt counting rate caused by turbidity, a and b experimental coefficients). A new turbidimetry method is based on this relation and discussed here. In situ measurements have been conducted with field gamma scintillometers and compared with different turbidimetry methods, such as nephelometry, gravi-metry and gamma ray absorptiometry. This method has been particularly convenient to continuous in situ measurements of turbidity from 0.5 g to several hundreds of g/1. This broad range is very well adapted to turbidity studies in estuaries.



The new method has been applied to study suspended matter dynamics in the Gironde estuary (France) during November 1968, as a part of a more general study of natural radioactivity in terrigenous sediments brought into the ocean.

M. Heuzel

Papers

A study of the dynamics of suspended matter by means of natural radioactive tracers: an application to the gironde estuary